Macro map 

macro_rules! map {
    { $first_key:expr => $first_value:expr $(, $key:expr => $value:expr)* $(,)? } => { ... };
    {} => { ... };
}

General-purpose macro for creating a map with keys pointing to values

let map: std::collections::HashMap<_, _> = map! {
    1 => 2,
    2 => 3,
    3 => 4
};

map! {} relies on type inference to determine which map to become.

See the crate-level documentation for more info.